Threats to Defense Department personnel and facilities increasingly are coming from trusted insiders, and to defeat them the Pentagon must beef up security from within, according to several reviews triggered by last year’s Washington Navy Yard killings.

The reviews say the shooting by a Navy contractor could have been prevented if the company that employed Aaron Alexis told the Navy about problems it was having with him in the months before he gunned down 12 civilian workers.

An independent study and an internal review ordered after the September 2013 massacre and released Tuesday said the Pentagon must expand its focus beyond defending against external threats. More attention must be paid, they concluded, to defending against threats from inside the workforce.

“For decades, the department has approached security from a perimeter perspective,” said Paul Stockton, former Pentagon assistant secretary for homeland defense and one of the authors of the independent review. “That approach is outmoded, it’s broken, and the department needs to replace it.”

According to the Navy probe, the Fort Lauderdale, Fla.-based company, The Experts, pulled Alexis’ access to classified material because of concerns he was having mental health problems. It then restored his access two days later and never told the Navy about it.

Alexis, a former Navy reservist, was shot to death during the incident.

The broader department reviews reached similar conclusions. They said the department should cut the number of workers who hold security clearances, conduct better and routinely updated background checks, and establish a system to evaluate and handle employees who are potential threats.

Preventing violence in the workplace must start “long before someone enters an installation with a weapon,” the internal review said.

The Navy investigation placed the most blame against Alexis’ employers.

The report written by Navy Adm. John Richardson said Alexis’s behavior raised concerns among his supervisors and others and indicated he may harm others. Had such information been reported to the government and acted upon, it stated, Alexis’ authorization to secure facilities would have been revoked.

Alexis’ company temporarily withdrew his access to classified information after a series of bizarre complaints and police incidents last August during a business trip to Newport, R.I. Alexis complained that people were following him, making noise and using a microwave machine to “send vibrations through the ceiling” in his hotel room.

Defense Secretary Chuck Hagel said Tuesday the department will set up an automated program that will continuously pull information from law enforcement and other databases. It will send out alerts if damaging information about a security-cleared worker is discovered.

Hagel said an inside threat management center will analyze the automatic record checks and “help connect the dots.” He said he will consider cutting the number of workers with clearances — currently about 2.5 million — by at least 10 percent.